The present invention relates to a dry etching apparatus and method using a reactive gases.
In order to fabricate very large-scale integrated, circuits, it is necessary to etch films of silicon, silicon nitride, tungsten, aluminum, etc. Parallel-plate plasma type etching apparatus are now widely used for this purpose. This plasma etching apparatus includes a vacuum container containing the workpiece and an anode electrode positioned opposite to a cathode electrode. A glow discharge plasma is produced upon introducing a feedstock gas into the container and applying radiofrequency energy between the anode and the cathode to generate reactive ions for the etching process. Positive ions in the plasma are accelerated by the cathode, which thereby bombard the workpiece. Thus, the workpiece is etched by the accelerated ions. When the material to be etched is, for example, polyersystalline silicon doped with phosphorus, it is possible to use chlorine as the feedstock gas. According to C. J. Mogab and H. J. Levinstein; "Journal of Vacuum Science Technology," pp. 721, 1980, anisotropic etching can be effectively performed by introducing C.sub.2 F.sub.6, for example, as an additive gas into the plasma vacuum container. In this case, a recombination action occurs of C1 radicals, resulting from dissociation of the introduced chlorine gas, and CF.sub. 3 radicals resulting from dissociation of the C.sub.2 F.sub.6 additive gas. Lateral etching of the side walls of the etched portion of the workpiece is prevented to some extent. However, this system does not permit optimizing, selecting and accurate control of both etching and anisotropy.
Moreover, this etching apparatus has several disadvantages. Since the workpiece to be etched is exposed to plasma, the workpiece undergoes various types of radiation damage. One form of radiation damage is caused by charged particles such as ions and electrons which electrify the workpiece. Because of this electrification when fabricating transistors, a thin oxide film is formed on the workpiece; the film is subjected to dielectric breakdown or traps are created in it. Another form of radiation damage is caused by soft x-rays which produce a shift in threshold value of the formed transistors. Another form of radiation damage is caused by positive ions in the plasma which are attracted toward the grounded evacuated container. These ions bombard and spatter the metal wall of the container, resulting in contamination of its interior.
In view of these problems an improved apparatus is disclosed in H. Akiya; "Proceedings of the third Symposium on Dry Processes," pp. 119, 1981. In that apparatus, neutral fluorine radicals having a kinetic energy equivalent to the plasma temperature are extracted from the plasma and directed onto the workpiece to effect etching. Another improved apparatus is disclosed in H. Okano et al.; "Proceedings of the Fourth Symposium on Dry Processes," pp. 6-10, 1982. In that apparatus, an excited gas, such as C1.sub.2 gas, is introduced into an evacuated container containing the workpiece; the workpiece is then illuminated with ultraviolet rays to cause a photochemical reaction to produce the etching action.
When a workpiece was selectively etched using these apparatuses, any aforementioned radiation damage was not observed. Rather, so-called side (or lateral) etching occurred and not the desired anisotropic etching. Side etching causes non-uniform characteristics in the resulting product. This is illustrated in FIG. 1. Shown is a workpiece 1 containing a silicon substrate 2 with an oxide film 3 formed thereon. A phosphorus doped polycrystalline silicon film 4 is formed on film 3 which will be etched as shown by the broken line. Silicon film 4 is coated with a resist film 5. Workpiece 1 is exposed to the feedstock gas (e.g. chlorine) and is then illuminated with ultraviolet rays. As a result, activated chlorine is produced for etching the workpiece 1 in a vertical direction as well as in a lateral direction. As shown, the side wall of silicon film 4 is etched off to form the undesirable shape indicated by numeral 7. Consequently, the desired mask configuration is not maintained and non-uniform circuit characteristics are produced.